This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to imaging of short T.sub.2 species by using magnetization transfer from the short T.sub.2 species to a long T.sub.2 species.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W.V. House, pp. 1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images A number of two-and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine," Scientific American, May 1982, pp. 78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.0, of the precession of the nuclei is the product of the magnetic field B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defined Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
The imaging of species having short spin-spin (T.sub.2) relaxation times can be difficult or impossible using direct detection of free induction decay (FID) signals from the species. However, a technique using magnetization transfer from a short T.sub.2 species to a long T.sub.2 species has been used to indirectly image the short T.sub.2 species.
These magnetization transfers or exchanges appear to be present in a large number of tissues and are thought to be related to the establishment of an exchangeable separate spin environment by macromolecules. The early work performed in spectrometers on hydrated protein samples as well as ex vivo samples of biological tissues established the exchange of magnetization between pools of relatively mobile long T.sub.2 species and more restricted short T.sub.2 species. Further more this phenomenon is shown to affect the macroscopically measured bulk relaxation times. A number of the essential elements of the magnetization transfer phenomenon have been described including the possible exchange mechanisms, rate of exchange, macroscopic relaxation times as well as estimates of the amount of exchangeable protons in various tissues.
S. Wolff and R. Balaban, "Magnetic Resonance in Medicine, 10(1), 135-144, (1989), first produced in vivo images with magnetization transfer weighted contrast (MTC). They also coined the terms free (H.sub.f) and restricted (H.sub.r) proton pools to describe the exchange compartments. Their technique took advantage of the broad lineshape of the short T.sub.2 species by performing continuous irradiation several kHz off resonance to achieve selective saturation. Sufficient saturation of the short T.sub.2 species, normally unobservable, is then indirectly observed via exchange with and subsequent partial saturation of the longer T.sub.2 species. However, continuous off resonance irradiation leads to the practical problems of high power deposition (SAR) and the need for auxiliary RF amplifiers on conventional whole body imagers. W. Dixon, et al., "Magnetic Resonance Imaging," 8(4), 417-422, (1990), has demonstrated the presence of magnetization transfer in conventional whole body imagers by the application of multiple off resonance pulses, however, the effects obtained were small.